Table 1_Soil quality enhancement drives tree growth and broadleaf dominance in fir-broadleaf mixed plantations.docx

IntroductionEvaluating soil quality is essential for guiding reforestation and land management strategies, particularly in degraded Chinese fir plantations where long-term productivity and successional dynamics remain poorly understood.
MethodsThis study assessed ten mixed-species planting patterns to quantify the Soil Quality Index (SQI) using a Minimum Data Set (MDS) approach, which reduces data redundancy by statistically identifying key indicators from a larger dataset, thereby effectively capturing essential soil functions, and subsequently explored the relationships between SQI and stand growth, structural diversity, biomass, net primary productivity (NPP), as well as percentage of broadleaf species (PBS).
ResultsSignificant differences were observed across planting patterns in diameter at breast height (DBH), tree height (TH), stand biomass (FB), structural diversity (variation in DBH [CVD] and Gini coefficient [GiniD]), and PBS. Soil properties—including physical (soil moisture), chemical (soil organic carbon [SOC], total nitrogen [TN], total phosphorus [TP], ammonium nitrogen [NH₄⁺], nitrate nitrogen [NO₃⁻], available phosphorus [AP]), microbial (microbial biomass carbon [MBC], nitrogen [MBN], and phosphorus [MBP]), and enzymatic (e.g., peroxidase [POD], alkaline phosphatase [ALP], urease [URE])—also varied significantly. SQI values ranged from 0.42 to 0.65, with patterns Fir–Mytilaria laosensis mixed (ML), Fir–Castanopsis hystrix mixed (CH), Fir–Michelia chapensis mixed (MC), and Fir–Schima superba mixed (SS) associated with both high SQI and greater biomass. Sensitivity analysis identified Fir–Cinnamomum porrectum mixed (CP), ML, and SS as particularly responsive to hybridization. Among soil factors, URE, AP, and MBC were key drivers of productivity, while URE, AP, MBC, and POD significantly predicted the proportion of broadleaf trees. Enhanced soil quality was positively associated with increases in DBH, TH, and PBS, accelerating the successional transition from fir-dominated to broadleaf-dominated stands. However, SQI was not significantly correlated with structural diversity metrics.
DiscussionThese results underscore the importance of rational species selection in restoring degraded fir plantations and demonstrate that improving soil quality is a critical mechanism promoting near-natural forest succession.

## 引言
评估土壤质量对于指导造林与土地管理策略至关重要，尤其针对退化杉木人工林——这类林分的长期生产力与演替动态目前仍尚未得到充分认知。

## 研究方法
本研究针对10种混交种植模式开展评估，采用最小数据集（Minimum Data Set, MDS）方法量化土壤质量指数（Soil Quality Index, SQI）：该方法通过统计学手段从大规模数据集里筛选关键指标以减少数据冗余，从而有效表征核心土壤功能；随后本研究进一步探究了土壤质量指数与林分生长、结构多样性、生物量、净初级生产力（Net Primary Productivity, NPP）以及阔叶树种占比（Percentage of Broadleaf Species, PBS）之间的关联。

## 研究结果
不同混交模式在胸径（diameter at breast height, DBH）、树高（tree height, TH）、林分生物量（stand biomass, FB）、结构多样性（胸径变异系数[CVD]与基尼系数[GiniD]）以及阔叶树种占比方面均存在显著差异。土壤属性同样存在显著差异，涵盖物理属性（土壤含水量）、化学属性（土壤有机碳[soil organic carbon, SOC]、全氮[total nitrogen, TN]、全磷[total phosphorus, TP]、铵态氮[ammonium nitrogen, NH₄⁺]、硝态氮[nitrate nitrogen, NO₃⁻]、有效磷[available phosphorus, AP]）、微生物属性（微生物量碳[microbial biomass carbon, MBC]、微生物量氮[microbial biomass nitrogen, MBN]与微生物量磷[microbial biomass phosphorus, MBP]）以及酶学属性（如过氧化物酶[peroxidase, POD]、碱性磷酸酶[alkaline phosphatase, ALP]、脲酶[urease, URE]）。土壤质量指数值介于0.42至0.65之间，其中杉木–壳菜果混交（ML）、杉木–红锥混交（CH）、杉木–乐昌含笑混交（MC）以及杉木–木荷混交（SS）模式兼具较高土壤质量指数与更大林分生物量。敏感性分析显示，杉木–天竺桂混交（CP）、ML以及SS模式对混交处理的响应尤为显著。在土壤因子中，脲酶、有效磷与微生物量碳是林分生产力的关键驱动因子，而脲酶、有效磷、微生物量碳与过氧化物酶可显著预测阔叶树种占比。土壤质量提升与胸径、树高以及阔叶树种占比的增加呈显著正相关，可加速杉木主导林相向阔叶树主导林相的演替过渡。但土壤质量指数与结构多样性指标并无显著相关性。

## 讨论
本研究结果凸显了退化杉木人工林修复中合理选择树种的重要性，并证实提升土壤质量是促进近自然森林演替的关键机制。